This invention relates to low pressure reforming. In particular, this invention relates to low pressure reforming of a full-boiling-range hydrocarbon comprising naphthenes and paraffins boiling primarily in the gasoline of naphtha range. The low pressure reforming process described in detail below comprises at least two reforming zones each having a separate recycle gas system enabling process operations in the second zone to be conducted at higher than normal C.sub.5 + reformate selectivity, especially when reforming naphthas containing relatively high amounts of C.sub.6 and C.sub.7 paraffins.
Catalytic reforming of hydrocarbon naphtha feeds to produce high octane gasoline products was developed as a relatively high pressure process. High pressures, greater than 350 psig, helped to limit those reactions such as condensation and polymerization, which resulted in coke formation on the catalyst. As a consequence, high pressure reforming permitted long on-stream periods of operation between replacement or regeneration of the catalyst. However, there are disadvantages to high pressure reforming. At high pressures, reactions which adversely affect the yield of valuable C.sub.5 + products are more prevalent. In particular, in high pressure reforming hydrocracking reactions are favored, thus producing less valuable light gases.
In response to the disadvantages of high pressure reforming several low pressure reforming processes have been developed and are commercially available. In particular, development of regenerative bimetallic catalysts, for example, platinum-rhenium catalysts, allows the use of reforming pressures well below 350 psig. Using these bimetallic catalysts, e.g., Pt-Re in semi-regenerative (long cycle) reformers, pressures in the vicinity of 200 psig are commonly employed. In addition, the use of continuous regeneration and swing reactor schemes permit operation substantially below 200 psig with the relatively short cycles between regenerations. In order to control catalyst fouling at low pressures, high hydrogen to hydrocarbon mole ratios have been suggested. However, a high hydrogen to hydrocarbon mole ratio requires extra gas handling equipment, for instance, very large recycle gas compressors. Moreover, operating at pressures below about 150 psig can require booster compressors to increase the pressure of the net hydrogen gas produced in order to transport it to other units. Accordingly, low pressure reforming loses some of its economic advantage due to frequent regeneration or high capital investment costs. U.S. Pat. No. 3,716,477 which issued Feb. 13, 1973 to R. L. Jacobson and R. D. Vanselow describes previous attempts to provide an attractive low pressure reforming process using a regenerative bimetallic catalyst.
One process configuration which has been suggested for low pressure reforming is known as "pressure-step" reforming. In this configuration two or more reforming zones are closely coupled, in series, with each subsequent zone operated at a pressure below that of the preceding zone. U.S. Pat. No. 4,002,555 which issued on Jan. 11, 1977 to R. A. Farnham describes a pressure-step reforming process. In the configuration suggested in this patent the hydrocarbon feedstock is separated into two fractions, a high-boiling fraction and a low-boiling fraction. The fractions are contacted with hydrogen in two separate reforming zones. The high-boiling fraction is processed in a first zone operated at a relatively high pressure, above 175 psig, and the low-boiling fraction is processed in a second zone operated at a lower pressure, below 150 psig. Recycle gas from the first zone is circulated to both zones using a single compressor.
Another process configuration which has been suggested for low pressure reforming is known as "swing reactor" or "cyclic regeneration" reforming. As the names imply, this configuration employs two or more parallel reactors, any one of which can be taken off line for catalyst regeneration without shutting down the unit.
The present invention provides an improved low pressure reforming process which advantageously combines the benefits of pressure-step reforming and swing reactor reforming.